Legal eagles..

My father in law (labourer) has worked last 14 years 5 days a week for a private organisation in England. I’ve seen his contract and it stipulates he only has 12 days annual leave plus 8 days public holidays every year, and that’s been his terms for 14 years.

I’m certain this is illegal after looking at Gov, ACAS websites etc

Anyone have any knowledge of similar things that have gone to court, likely payouts etc ?

Cheers for any help and PMs open and also welcomed.